1. Field of the Invention
The present invention relates broadly to the art of making safety glass and more particularly to the art of making safety glass windshields combining improvements in safety performance and optical quality durability.
2. Description of the Prior Art
Safety glass is a well-known term describing a glass-plastic laminate designed to reduce the severity of lacerative injuries resulting from impact sufficient to break the glass. A plastic film is laminated to a glass sheet so that upon impact sufficient to break the glass, the film adheres to the glass fragments, thus minimizing their dispersion. To be useful as safety glass, a laminate must have the following properties over a wide range of temperature and moisture conditions: (1) high energy absorption to minimize concussive injuries on impact, (2) high shear and tear strength to prevent rupture of the film by glass fragments, (3) sufficient adhesion between the layers to minimize dispersion of glass fragments thereby reducing the potential for lacerative injury, and (4) good optical quality.
Safety glass commercially employed is commonly a multiple laminate of two plies of glass with an interlayer of polyvinyl butyral plastic film. Alternative safety glass laminates, particularly for use as automobile windshields, are proposed either in the form of a single ply of glass with a plastic innerlayer or in the form of standard commercial safety glass with a plastic innerlayer on the inboard ply of glass. Upon impact sufficient to break these alternative forms of windshields, the probability of encountering glass fragments inside the passenger compartment is reduced. However, since the innerlayer will be exposed, the demands on the plastic film are much greater. It must not only meet the requirements of energy absorption, tear strength, adherence and optical quality previously discussed, but its surface must also have good weathering properties, chemical stability and abrasion resistance to provide durability for its required optical quality.
Thermoset polymer films typically have such durability; however, such films are difficult to laminate. In addition, rigid films contribute to concussive injury. Thermoplastic polymer films are relatively easy to laminate and are sufficiently ductile to absorb energy on impact, but they are often moisture and solvent sensitive and their surfaces are easily scratched, resulting in a loss of optical transparency.
It is known in the polymer art that some thermoplastic polymers, if crosslinked, become less elastomeric, less soluble, and in many respects similar to thermoset polymers. Thermoplastic polymers may be crosslinked during the polymerization reaction. However, crosslinked polymers are difficult to process. Alternatively, a polymer formulation may be treated with a crosslinking agent which can be activated subsequent to forming the polymer into the desired shape. Photochemical activation of a photosensitive crosslinking agent is an example.
The photochemical properties of benzophenone are well-known. For example, when exposed to ultraviolet radiation, such as sunlight, benzophenone in solution in an organic medium such as isopropanol, produces tetraphenylethylene glycol. The reaction mechanism consists of the following sequence of steps: ##STR1##
Excited benzophenone molecules are equally capable of abstracting hydrogen from hydrocarbons. The well-known oxidative dimerization of aliphatic hydrocarbons by photochemical reaction with benzophenone proceeds as follows: ##STR2##
A method for decreasing the thermoplasticity and solubility of polymers of ethylene usable as safety glass interlayers is taught by Roedel in U.S. Pat. No. 2,484,529. The method involves blending such polymers with ketones such as acetone, benzophenone or benzoin, and then exposing the blend to ultraviolet light.
An improved method for making polyethylene articles is taught by Tocker in U.S. Pat. No. 3,214,492. The method involves copolymerizing ethylene with acryloxy- or methacryloxy- substituted benzophenones or acetophenones, shaping the copolymer into a useful article, and exposing the article to ultraviolet radiation to produce crosslinking.
In U.S. Pat. No. 3,219,566, Potts et al teach that anthrone is superior to benzophenone for crosslinking polyethylene and polypropylene in the presence of ultraviolet light.
A method for making crosslinkable polyesters is taught in U.S. Pat. No. 3,518,175 to Bell. The method involves copolymerizing the polyester with a substituted benzophenone. Exposure of the resulting photosensitized terpolymer to ultraviolet radiation produces crosslinking.
In U.S. Pat. No. 2,929,710, Martin discloses polyvinyl acetal films containing initiators activatable by light of wavelengths from 1800 to 7000 Angstroms. The polyvinyl acetals of Martin's invention always contain a plurality of vinylidene groups in conjugative double bond systems.
It is not shown or suggested in the polymer art that saturated polyvinyl acetals, that is, polyvinyl acetals without extralinear vinylidene groups, are likewise crosslinkable by exposure to ultraviolet light in the presence of a photoinitiator such as benzophenone to produce a transparent layer that is crosslinked in at least a surface portion to improve its solvent-abrasion resistance without degrading its desired mechanical, adhesive and optical properties so that the polymer may be employed as an exposed innerlayer in a safety glass laminate.